Fe‐Doped Ni3C Nanodots in N‐Doped Carbon Nanosheets for Efficient Hydrogen‐Evolution and Oxygen‐Evolution Electrocatalysis

Uniform Ni₃C nanodots dispersed in ultrathin N‐doped carbon nanosheets were successfully prepared by carburization of the two dimensional (2D) nickel cyanide coordination polymer precursors. The Ni₃C based nanosheets have lateral length of about 200 nm and thickness of 10 nm. When doped with Fe, the Ni₃C based nanosheets exhibited outstanding electrocatalytic properties for both the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). For example, 2 at % Fe (atomic percent) doped Ni₃C nanosheets depict a low overpotential (292 mV) and a small Tafel slope (41.3 mV dec⁻¹) for HER in KOH solution. An outstanding OER catalytic property is also achieved with a low overpotential of 275 mV and a small Tafel slope of 62 mV dec⁻¹ in KOH solution. Such nanodot‐incorporated 2D hybrid structures can serve as an efficient bifunctional electrocatalyst for overall water splitting.     

Suppression of the Charge Density Wave State in Two‐Dimensional 1T‐TiSe2 by Atmospheric Oxidation

Two‐dimensional (2D) metallic transition‐metal dichalcogenides (TMDCs), such as 1T ‐TiSe₂, have recently emerged as unique platforms for exploring their exciting properties of superconductivity and the charge density wave (CDW). 2D 1T ‐TiSe₂ undergoes rapid oxidation under ambient conditions, significantly affecting its CDW phase‐transition behavior. We comprehensively investigate the oxidation process of 2D TiSe₂ by tracking the evolution of the chemical composition and atomic structure with various microscopic and spectroscopic techniques and reveal its unique selenium‐assisting oxidation mechanism. Our findings facilitate a better understanding of the chemistry of ultrathin TMDCs crystals, introduce an effective method to passivate their surfaces with capping layers, and thus open a way to further explore the functionality of these materials toward devices.     

Direct Monitoring of Cell Membrane Vesiculation with 2D AuNP@MnO2 Nanosheet Supraparticles at the Single‐Particle Level

We herein demonstrate robust two‐dimensional (2D) UFO‐shaped plasmonic supraparticles made of gold nanoparticles (AuNPs) and MnO₂ nanosheets (denoted as AMNS‐SPs) for directly monitoring cell membrane vesiculation at the single‐particle level. Because the decorated MnO₂ nanosheets are ultrathin (4.2 nm) and have large diameters (230 nm), they are flexible enough for deformation and folding for parceling of the AuNPs during the endocytosis process. Correspondingly, the surrounding refractive index of the AuNPs increases dramatically, which results in a distinct red‐shift of the localized surface plasmon resonance (LSPR). Such LSPR modulation provides a convenient and accurate means for directly monitoring the dynamic interactions between 2D nanomaterials and cell membranes. Furthermore, for the endocytosed AMNS‐SPs, the subsequent LSPR blue‐shift induced by etching effects of reducing molecules is promising for exploring the local environment redox states at the single‐cell level.     

Generalized Self‐Doping Engineering towards Ultrathin and Large‐Sized Two‐Dimensional Homologous Perovskites

Two‐dimensional (2D) homologous perovskites are arousing intense interest in photovoltaics and light‐emitting fields, attributing to significantly improved stability and increasing optoelectronic performance. However, investigations on 2D homologous perovskites with ultrathin thickness and large lateral dimension have been seldom reported, being mainly hindered by challenges in synthesis. A generalized self‐doping directed synthesis of ultrathin 2D homologous (BA)₂(MA)_{n −1}Pb_n Br_{3n +1} (1<n <∞) perovskites uses 2D (BA)₂PbBr₄ perovskites as the template with MA⁺ dopant. Ultrathin (BA)₂(MA)_{n −1}Pb_n Br_{3n +1} perovskites are formed via an intercalation–merging mechanism, with thickness shrinking down to 4.2 nm and the lateral dimension to 57 μm. The ultrathin 2D homologous (BA)₂(MA)_{n −1}Pb_n Br_{3n +1} perovskites are potential materials for photodetectors with promising photoresponse and stability.     

Electrophoretic Deposition of Binder‐Free MnO2/Graphene Films for Lithium‐Ion Batteries

Binder‐free, nano‐sized needlelike MnO₂ ‐submillimeter‐sized reduced graphene oxide (nMnO₂‐srGO ) hybrid films with abundant porous structures were fabricated through electrophoretic deposition and subsequent thermal annealing at 500 °C for 2 h. The as‐prepared hybrid films exhibit a unique hierarchical morphology, in which nMnO₂ with a diameter of 20—50 nm and a length of 300—500 nm is randomly anchored on both sides of srGO . When evaluated as binder‐free anodes for lithium‐ion half‐cell, the nMnO₂‐srGO composites with a content of 76.9 wt% MnO₂ deliver a high capacity of approximately 1652.2 mA •h•g⁻¹ at a current density of 0.1 A•g⁻¹ after 200 cycles. The high capacity remains at 616.8 mA •h•g⁻¹ (ca. 65.1% capacity retention) at a current density as high as 4 A•g⁻¹. The excellent electrochemical performance indicates that the nMnO₂‐srGO hybrid films could be a promising anode material for lithium ion batteries (LIBs ).     

Unveiling the Layer‐Dependent Catalytic Activity of PtSe2 Atomic Crystals for the Hydrogen Evolution Reaction

Two‐dimensional (2D) PtSe₂ shows the most prominent layer‐dependent electrical properties among various 2D materials and high catalytic activity for hydrogen evolution reaction (HER), and therefore, it is an ideal material for exploring the structure–activity correlations in 2D systems. Here, starting with the synthesis of single‐crystalline 2D PtSe₂ with a controlled number of layers and probing the HER catalytic activity of individual flakes in micro electrochemical cells, we investigated the layer‐dependent HER catalytic activity of 2D PtSe₂ from both theoretical and experimental perspectives. We clearly demonstrated how the number of layers affects the number of active sites, the electronic structures, and electrical properties of 2D PtSe₂ flakes and thus alters their catalytic performance for HER. Our results also highlight the importance of efficient electron transfer in achieving optimum activity for ultrathin electrocatalysts. Our studies greatly enrich our understanding of the structure–activity correlations for 2D catalysts and provide new insight for the design and synthesis of ultrathin catalysts with high activity.     

Coaxial Carbon/MnO2 Hollow Nanofibers as Sulfur Hosts for High‐Performance Lithium‐Sulfur Batteries

Lithium‐sulfur (Li‐S) batteries have recently attracted a large amount of attention as promising candidates for next‐generation high‐power energy storage devices because of their high theoretical capacity and energy density. However, the shuttle effect of polysulfides and poor conductivity of sulfur are still vital issues that constrain their specific capacity and cyclic stability. Here, we design coaxial MnO₂‐graphitic carbon hollow nanofibers as sulfur hosts for high‐performance lithium‐sulfur batteries. The hollow C/MnO₂ coaxial nanofibers are synthesized via electrospinning and carbonization of the carbon nanofibers (CNFs), followed by an in situ redox reaction to grow MnO₂ nanosheets on the surface of CNFs. The inner graphitic carbon layer not only maintains intimate contact with sulfur and outer MnO₂ shell to significantly increase the overall electrical conductivity but also acts as a protective layer to prevent dissolution of polysulfides. The outer MnO₂ nanosheets restrain the shuttle effect greatly through chemisorption and redox reaction. Therefore, the robust S@C/MnO₂ nanofiber cathode delivers an extraordinary rate capability and excellent cycling stability with a capacity decay rate of 0.044 and 0.051 % per cycle after 1000 cycles at 1.0 C and 2.0 C, respectively. Our present work brings forward a new facile and efficient strategy for the functionalization of inorganic metal oxide on graphitic carbons as sulfur hosts for high performance Li‐S batteries.     

Ultrathin FeSe2 Nanosheets: Controlled Synthesis and Application as a Heterogeneous Catalyst in Dye‐Sensitized Solar Cells

Two‐dimensional (2D) semiconducting nanosheets have emerged as an important field of materials, owing to their unique properties and potential applications in areas ranging from electronics to catalysis. However, the controlled synthesis of ultrathin 2D nanosheets remains a great challenge, due to the lack of an intrinsic driving force for anisotropic growth. High‐quality ultrathin 2D FeSe₂ nanosheets with average thickness below 7 nm have been synthesized on large scale by a facile solution method, and a formation mechanism has been proposed. Due to their favorable structural features, the as‐synthesized ultrathin FeSe₂ nanosheets exhibit excellent electrocatalytic activity for the reduction of triiodide to iodide and low charge‐transfer resistance at the electrolyte–electrode interface in dye‐sensitized solar cells (DSSCs). The DSSCs with FeSe₂ nanosheets as counter electrode material achieve a high power conversion efficiency of 7.53 % under a simulated solar illumination of 100 mW cm^?2 (AM 1.5), which is comparable with that of Pt‐based devices (7.47 %).     

Synthesis of Sub‐2 nm Iron‐Doped NiSe2 Nanowires and Their Surface‐Confined Oxidation for Oxygen Evolution Catalysis

Ultrathin nanostructures are attractive for diverse applications owing to their unique properties compared to their bulk materials. Transition‐metal chalcogenides are promising electrocatalysts, yet it remains difficult to make ultrathin structures (sub‐2 nm), and the realization of their chemical doping is even more challenging. Herein we describe a soft‐template mediated colloidal synthesis of Fe‐doped NiSe₂ ultrathin nanowires (UNWs) with diameter down to 1.7 nm. The synergistic interplay between oleylamine and 1‐dodecanethiol is crucial to yield these UNWs. The in situ formed amorphous hydroxide layers that is confined to the surface of the ultrathin scaffolds enable efficient oxygen evolution electrocatalysis. The UNWs exhibit a very low overpotential of 268 mV at 10 mA cm^?2 in 0.1 m KOH, as well as remarkable long‐term stability, representing one of the most efficient noble‐metal‐free catalysts.     

Two‐Dimensional Layered Zinc Silicate Nanosheets with Excellent Photocatalytic Performance for Organic Pollutant Degradation and CO2 Conversion

Two‐dimensional (2D) photocatalysts are highly attractive for their great potential in environmental remediation and energy conversion. Herein, we report a novel layered zinc silicate (LZS) photocatalyst synthesized by a liquid‐phase epitaxial growth route using silica derived from vermiculite, a layered silicate clay mineral, as both the lattice‐matched substrate and Si source. The epitaxial growth of LZS is limited in the 2D directions, thus generating the vermiculite‐type crystal structure and ultrathin nanosheet morphology with thicknesses of 8–15 nm and a lateral size of about 200 nm. Experimental observations and DFT calculations indicated that LZS has a superior band alignment for the degradation of organic pollutants and reduction of CO₂ to CO. The material exhibited efficient photocatalytic performance for 4‐chlorophenol (4‐CP) degradation and CO₂ conversion into CO and is the first example of a claylike 2D photocatalyst with strong photooxidation and photoreduction capabilities.     

A Zero‐Dimensional Organic Seesaw‐Shaped Tin Bromide with Highly Efficient Strongly Stokes‐Shifted Deep‐Red Emission

The synthesis and characterization is reported of (C₉NH₂₀)₂SnBr₄, a novel organic metal halide hybrid with a zero‐dimensional (0D) structure, in which individual seesaw‐shaped tin (II) bromide anions (SnBr₄^2?) are co‐crystallized with 1‐butyl‐1‐methylpyrrolidinium cations (C₉NH₂₀⁺). Upon photoexcitation, the bulk crystals exhibit a highly efficient broadband deep‐red emission peaked at 695 nm, with a large Stokes shift of 332 nm and a high quantum efficiency of around 46 %. The unique photophysical properties of this hybrid material are attributed to two major factors: 1) the 0D structure allowing the bulk crystals to exhibit the intrinsic properties of individual SnBr₄^2? species, and 2) the seesaw structure enabling a pronounced excited state structural deformation as confirmed by density functional theory (DFT) calculations.     

Superior Electrical Conductivity in Hydrogenated Layered Ternary Chalcogenide Nanosheets for Flexible All‐Solid‐State Supercapacitors

As the properties of ultrathin two‐dimensional (2D) crystals are strongly related to their electronic structures, more and more attempts were carried out to tune their electronic structures to meet the high standards for the construction of next‐generation smart electronics. Herein, for the first time, we show that the conductive nature of layered ternary chalcogenide with formula of Cu₂WS₄ can be switched from semiconducting to metallic by hydrogen incorporation, accompanied by a high increase in electrical conductivity. In detail, the room‐temperature electrical conductivity of hydrogenated‐Cu₂WS₄ nanosheet film was almost 10¹⁰ times higher than that of pristine bulk sample with a value of about 2.9×10⁴ S m^?1, which is among the best values for conductive 2D nanosheets. In addition, the metallicity in the hydrogenated‐Cu₂WS₄ is robust and can be retained under high‐temperature treatment. The fabricated all‐solid‐state flexible supercapacitor based on the hydrogenated‐Cu₂WS₄ nanosheet film shows promising electrochemical performances with capacitance of 583.3 F cm^?3 at a current density of 0.31 A cm^?3. This work not only offers a prototype material for the study of electronic structure regulation in 2D crystals, but also paves the way in searching for highly conductive electrodes.     

A Ternary Solvent Method for Large‐Sized Two‐Dimensional Perovskites

Recent reports demonstrate that a two‐dimensional (2D) structural characteristic can endow perovskites with both remarkable photoelectric conversion efficiency and high stability, but the synthesis of ultrathin 2D perovskites with large sizes by facile solution methods is still a challenge. Reported herein is the controlled growth of 2D (C₄H₉NH₃)₂PbBr₄ perovskites by a chlorobenzene‐dimethylformide‐acetonitrile ternary solvent method. The critical factors, including solvent volume ratio, crystallization temperature, and solvent polarity on the growth dynamics were systematically studied. Under optimum reaction condition, 2D (C₄H₉NH₃)₂PbBr₄ perovskites, with the largest lateral dimension of up to 40 μm and smallest thickness down to a few nanometers, were fabricated. Furthermore, various iodine doped 2D (C₄H₉NH₃)₂PbBr_x I_4−x perovskites were accessed to tune the optical properties rationally.     

Ultrathin Two‐Dimensional Organic–Inorganic Hybrid Perovskite Nanosheets with Bright,Tunable Photoluminescence and High Stability

Two‐dimensional (2D) organic–inorganic hybrid perovskite nanosheets (NSs) are attracting increasing research interest due to their unique properties and promising applications. Here, for the first time, we report the facile synthesis of single‐ and few‐layer free‐standing phenylethylammonium lead halide perovskite NSs, that is, (PEA)₂PbX₄ (PEA=C₈H₉NH₃, X=Cl, Br, I). Importantly, their lateral size can be tuned by changing solvents. Moreover, these ultrathin 2D perovskite NSs exhibit highly efficient and tunable photoluminescence, as well as superior stability. Our study provides a simple and general method for the controlled synthesis of 2D perovskite NSs, which may offer a new avenue for their fundamental studies and optoelectronic applications.     

Ultrathin Two‐Dimensional Organic–Inorganic Hybrid Perovskite Nanosheets with Bright,Tunable Photoluminescence and High Stability

Two‐dimensional (2D) organic–inorganic hybrid perovskite nanosheets (NSs) are attracting increasing research interest due to their unique properties and promising applications. Here, for the first time, we report the facile synthesis of single‐ and few‐layer free‐standing phenylethylammonium lead halide perovskite NSs, that is, (PEA)₂PbX₄ (PEA=C₈H₉NH₃, X=Cl, Br, I). Importantly, their lateral size can be tuned by changing solvents. Moreover, these ultrathin 2D perovskite NSs exhibit highly efficient and tunable photoluminescence, as well as superior stability. Our study provides a simple and general method for the controlled synthesis of 2D perovskite NSs, which may offer a new avenue for their fundamental studies and optoelectronic applications.     

Single Crystalline Na0.7MnO2 Nanoplates as Cathode Materials for Sodium‐Ion Batteries with Enhanced Performance

Single crystalline rhombus‐shaped Na_0.7MnO₂ nanoplates have been synthesized by a hydrothermal method. TEM and HRTEM analyses revealed that the Na_0.7MnO₂ single crystals predominantly exposed their (100) crystal plane, which is active for Na⁺‐ion insertion and extraction. When applied as cathode materials for sodium‐ion batteries, Na_0.7MnO₂ nanoplates exhibited a high reversible capacity of 163 mA h g^?1, a satisfactory cyclability, and a high rate performance. The enhanced electrochemical performance could be ascribed to the predominantly exposed active (100) facet, which could facilitate fast Na⁺‐ion insertion/extraction during the discharge and charge process.     

Amorphous MnO2 as Cathode Material for Sodium‐ion Batteries

Amorphous MnO₂ has been prepared from the reduction of KMnO₄ in ethanol media by a facile one‐step wet chemical route at room temperature. The electrochemical properties of amorphous MnO₂ as cathode material in sodium‐ion batteries (SIBs ) are studied by galvanostatic charge/discharge testing. And the structure and morphologies of amorphous MnO₂ are investigated by X‐ray diffraction (XRD ), scanning electron microscopy (SEM ), transmission electron microscopy (TEM ) and Raman spectra. The results reveal that as‐synthesized amorphous MnO₂ electrode material exhibits a spherical morphology with a diameter between 20 and 60 nm. The first specific discharge capacity of the amorphous MnO₂ electrode is 123.2 mAh •g⁻¹ and remains 136.8 mAh •g⁻¹ after 100 cycles at the current rate of 0.1 C. The specific discharge capacity of amorphous MnO₂ is maintained at 139.2, 120.4, 89, 68 and 47 mAh •g⁻¹ at the current rate of 0.1 C, 0.2 C, 0.5 C, 1 C and 2 C, respectively. The results indicate that amorphous MnO₂ has great potential as a promising cathode material for SIBs .     

Ultrathin Ca‐PO4‐CO3 Solid‐Solution Nanowires: A Controllable Synthesis and Full‐Color Emission by Rare‐Earth Doping

It was found that calcium carbonate (CaCO₃) and hydroxyapatite (Ca₁₀(OH)₂(PO₄)₆), which are two crucial constituents of the most abundant minerals in nature and very important bioinorganic components in the tissues of mineralizing organisms, can form solid solutions in a wide range of PO₄^3?/CO₃^2? (P/C) ratios at low temperature when prepared as ultrathin nanowire structures. This is due to the special reactivity of ultrasmall nanocrystals, which can effectively lower the synthetic temperature and promote the formation of solid solutions. The as‐prepared ultrathin nanowires with suitable P/C ratios presented strong blue luminescence due to the existence of abundant defects strengthened by CO₃^2?. If used as the matrix, the as‐prepared ultrathin nanowires demonstrated bright green or red luminescent properties when doped with Tb³⁺ or Eu³⁺ ions, and simultaneously retained their original morphologies. These three kinds of fluorescent nanowires could reproduce a full range of luminescence colors based on additive color mixtures of the three primary colors (red, green, and blue). In addition, under the same reaction system, ultrafine rare‐earth‐doped (Ce³⁺, Tb³⁺, Eu³⁺) nanowires (about 1 nm in diameter) were synthesized by using a one‐step hydrothermal process, which further pushed the size of the Ca‐PO₄‐CO₃ nanobuilding blocks to one unit cell region. These ultrafine nanowires displayed excellent film‐forming properties and the ability to absorb UV radiation.     

Ultrathin polymer gate buffer layer for air‐stable,low‐voltage,n‐channel organic thin‐film transistors

An ultrathin poly(methyl methacrylate) (PMMA) buffer layer was developed to improve the performance of n‐channel organic thin‐film transistors (OTFTs). The 8 nm‐thick PMMA film, prepared by spin‐coating, provided a very smooth surface and a uniform coverage on SiO₂ surface reproducibly, which was confirmed by X‐ray reflectivity (XR) measurement. Then, we fabricated N,N′‐ditridecyl‐3,4,9,10‐perylenetetracarboxylic diimide (PTCDI‐C13) thin‐film transistors with and without this 8 nm‐thick PMMA insulating layer on SiO₂ gate insulators and achieved one‐order increase of field‐effect mobility (up to 0.11 cm²/(Vs) in a vacuum), one‐half decrease of threshold voltage, and reduction of current hysteresis with the PMMA layer. Only TFTs with the PMMA layer displayed n‐channel operation in air and showed field‐effect mobility of 0.10 cm²/(Vs). We consider that electrical characteristics of n‐channel OTFTs were considerably improved because the ultrathin PMMA film could effectively passivate the SiO₂ insulator surface and decrease interfacial electron traps. This result suggests the importance of the ultrathin PMMA layer for controlling the interfacial state at the semiconductor/insulator interface and the device characteristics of OTFTs. Copyright © 2009 John Wiley & Sons, Ltd.     

Air‐Stable,Lead‐Free Zero‐Dimensional Mixed Bismuth‐Antimony Perovskite Single Crystals with Ultra‐broadband Emission

Lead‐free zero‐dimensional (0D) organic‐inorganic metal halide perovskites have recently attracted increasing attention for their excellent photoluminescence properties and chemical stability. Here, we report the synthesis and characterization of an air‐stable 0D mixed metal halide perovskite (C₈NH₁₂)₄Bi_0.57Sb_0.43Br₇?H₂O, in which individual [BiBr₆]^3? and [SbBr₆]^3? octahedral units are completely isolated and surrounded by the large organic cation C₈H₁₂N⁺. Upon photoexcitation, the bulk crystals exhibit ultra‐broadband emission ranging from 400 to 850 nm, which originates from both free excitons and self‐trapped excitons. This is the first example of 0D perovskites with broadband emission spanning the entire visible spectrum. In addition, (C₈NH₁₂)₄Bi_0.57Sb_0.43Br₇?H₂O exhibits excellent humidity and light stability. These findings present a new direction towards the design of environmentally‐friendly, high‐performance 0D perovskite light emitters.